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A plan to avert an offensive by the Assad government against the last major rebel-held area in Syria stumbled Monday when the main extremist group failed to leave a proposed buffer zone as part of a deal to demilitarize the region.

However, Russia may still prevent Syria’s President Bashar al-Assad from launching an assault to retake Idlib, a province in northwestern Syria, as Moscow now appears eager to avoid a fight and instead find a political settlement to end the Syrian war and cement its growing role as a power broker in the Middle East.

Russia and Turkey—which fears an attack on Idlib will send hundreds of thousands of civilians fleeing toward Turkish territory—agreed last month to create a roughly 10-mile buffer zone between territory controlled by opposition and government forces in Idlib. Thousands of rebels have fled to the province as Mr. Assad has gradually retaken a large portion of the country.

The deal required all heavy weaponry and radical militants to be moved by Monday out of the buffer zone, which will be supervised by Russian and Turkish troops.

The biggest threat to the demilitarized zone’s success has been the presence of designated terrorist groups such as Hayat Tahrir al-Sham, which evolved from the al Qaeda-affiliated Nusra Front. HTS is key to any deal, as it and other extremist groups control more than half of the province and the biggest part of the proposed buffer zone.

HTS in recent days has toned down its resistance to the deal, but has refused to give up fighting or disarm. By Monday’s deadline, its militants had not left the demarcated area, one of the conditions of the Russian-Turkish deal, according to the U.K.-based Syrian Observatory for Human Rights.

Russia and Turkey are, however, intent on making the agreement work. Last week, Russian Foreign Minister Sergei Lavrov said Moscow would allow for a couple of days’ delay in setting up the demilitarized zone.

Russia’s agreement with Turkey in Idlib has put it at odds with the Assad government. Mr. Assad has repeatedly vowed to retake control of the entire country, and has called the agreement between Russia and Turkey a “temporary measure” before returning Idlib to government control.

In a news conference Monday, Syrian Foreign Minister Walid al-Moallem said the national army was deployed near Idlib, ready to attack if the rebels didn’t withdraw.

The Syrian government, however, needs support from the Russian air force to retake Idlib.

Russia wants to prevent fresh fighting as it seeks to consolidate its status in the region, even if that means leaving parts of the country out of the control of its ally, Mr. Assad.

“Russia wants a diplomatic solution to seal its military success,” said Dmitri Trenin, director of the independent Carnegie Moscow Center. “Such a solution would mean some sort of power-sharing in Syria, or at least acknowledgment of spheres of influence,” he said.

However, Russia also wants to clear Idlib of groups it considers terrorists, and may still help the Syrian government to do so if a deal fails, as it has done repeatedly since intervening in the war in 2015.

Meanwhile, Turkey is currently deepening its presence in northwest Syria, while U.S.-backed Kurdish groups control an area in the northeast amounting to about a quarter of the country’s total territory.

In recent days, HTS has softened its opposition to the demilitarization plan and, according to a member of the group based in Idlib, is negotiating with Turkey as to whether to comply with the plan. But at the same time, it has refused to withdraw from the province and has maintained its call for a holy war against Mr. Assad and warned its followers against trusting the Russians.